1. Field of the Invention
The present invention relates to an optical wave interference measuring apparatus that radiates measurement light to a test surface and measures the shape of the test surface on the basis of an interference fringe formed by interference between reference light and returning light from the test surface, and more particularly, to an optical wave interference measuring apparatus suitable for a small test surface that is rotationally symmetric and has a complicated shape.
2. Description of the Related Art
A method has been proposed which radiates spherical waves to an aspheric test surface and specifies the shape of the test surface on the basis of an interference fringe formed by interference between the reference light and the returning light from the test surface. However, it is difficult to obtain an interference fringe corresponding to the entire test surface using this method.
Therefore, a method has been proposed which sequentially moves an interferometer or a test surface in a measurement optical axis direction to sequentially generate an interference fringe corresponding to each partial region of the test surface in the diametric direction thereof, analyzes each of the interference fringes to calculate the shape of each partial region of the test surface in the diametric direction, and connects the shapes to specify the shape of the entire test surface (see JP-A-62-126305).
In addition, a method has been proposed which sequentially moves an interferometer or a test surface in a plane perpendicular to the measurement optical axis, enlarges an interference fringe corresponding to each partial region of the test surface to a size that can be analyzed whenever the movement is performed, captures the enlarged interference fringes, analyzes each of the interference fringes to calculate the shape of each partial region of the test surface, and connects the shapes to specify the shape of the entire test surface (see U.S. Pat. No. 6,956,657).
In recent years, the shape of aspheric lenses has been complicated, and aspheric lenses have been used in which both a concave portion (concave surface portion) and a convex portion (convex surface portion) each having the optical axis (central axis) of the lens surface as its center are formed in one lens surface. It is difficult to measure the shape of the test surface having the concave surface portion and the convex surface portion using an optical interferometer. Therefore, the shape of the test surface has been measured by a three-dimensional shape measuring method using an optical probe.
For example, the reason why it is difficult to measure the shape of the test surface using the optical interferometer is that the concave surface portion and the convex surface portion have opposite gradients with respect to the optical axis of the test surface (when the test surface faces upward, the gradient of the concave surface portion is reduced toward the optical axis, and the gradient of the convex surface portion is increased toward the optical axis). That is, in a general optical interference measurement method, an appropriate interference fringe is obtained from only the region in which measurement light radiated to the test surface is retro-reflected from the test surface (returning light travels through the original optical path in the opposite direction). In the methods disclosed in JP-A-62-126305 and U.S. Pat. No. 6,956,657 in which the measurement light radiated to the test surface is fixed to a spherical wave that travels along the measurement optical axis while diverging or a spherical wave that travels along the measurement optical axis while converging, the returning light from the test surface travels in different directions in the concave surface portion and the convex surface portion. Therefore, it is difficult to obtain appropriate interference fringes from both the concave surface portion and the convex surface portion.
In addition, an interferometer using a plane wave as the measurement light has generally been known. In a test surface having a complicated shape, there is a large variation in the inclination of each partial region of the test surface. Therefore, when each partial region of the test surface is measured while the direction of measurement light radiated to the test surface is changed, the density of the obtained interference fringes is excessively high. When the test surface is small, the divergent angle of the reflected light of the measurement light radiated to the test surface is increased, and returning light related to the formation of the interference fringe does not satisfy the conditions of a paraxial beam. As a result, a large measurement error occurs.